1. Field of the Invention
The invention relates to methods and apparatus for controlling the operation of xe2x80x9clean-burnxe2x80x9d internal combustion engines used in motor vehicles to obtain improved engine and/or vehicle performance, such as improved vehicle fuel economy or reduced overall vehicle emissions.
2. Background Art
The exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituent gases, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and oxygen (O2). The respective rates at which an engine generates these constituent gases are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (xcex), engine speed and load, engine temperature, ambient humidity, ignition timing (xe2x80x9csparkxe2x80x9d), and percentage exhaust gas recirculation (xe2x80x9cEGRxe2x80x9d). The prior art often maps values for instantaneous engine-generated or xe2x80x9cfeedgasxe2x80x9d constituents, such as HC, CO and NOx, based, for example, on detected values for instantaneous engine speed and engine load.
To limit the amount of feedgas constituents that are exhausted through the vehicle""s tailpipe to the atmosphere as xe2x80x9cemissions,xe2x80x9d motor vehicles typically include an exhaust purification system having an upstream and a downstream three-way catalyst. The downstream three-way catalyst is often referred to as a NOx xe2x80x9ctrapxe2x80x9d. Both the upstream and downstream catalyst store NOx when the exhaust gases are xe2x80x9cleanxe2x80x9d of stoichiometry and release previously stored NOx for reduction to harmless gases when the exhaust gases are xe2x80x9crichxe2x80x9d of stoichiometry.
In accordance with one prior art method, the duration of a given lean operating excursion is controlled based upon an estimate of how much NOx has accumulated in the trap since the lean excursion began. For example, in one prior art system, a controller estimates instantaneous feedgas NOx and accumulates the estimates over time to obtain a measure representing total NOx generated during the lean excursion. The controller discontinues the lean operating excursion when the total generated-NOx measure exceeds a predetermined threshold representing the trap""s nominal NOx-storage capacity, usually set at a predetermined level below the saturation level of the trap. In this manner, the prior art seeks to discontinue lean operation before the trap is fully saturated with NOx, because feedgas NOx would otherwise pass through the trap and effect an increase in tailpipe NOx emissions.
A trap purge event is thereafter scheduled, during which the engine is operated with a xe2x80x9crichxe2x80x9d air-fuel mixture to release the stored NOx and rejuvenate the trap. Each purge event is characterized by a xe2x80x9cfuel penaltyxe2x80x9d consisting generally of an amount of fuel required to release both the oxygen stored in the three-way catalyst, and the oxygen and NOx stored in the trap. Significantly, the trap""s NOx-storage capacity is known to decline in a generally-reversible manner over time due to sulfur poisoning or xe2x80x9csulfurization,xe2x80x9d and in a generally-irreversible manner over time due, for example, to component xe2x80x9cagingxe2x80x9d from thermal effects and xe2x80x9cdeep-diffusionxe2x80x9d/xe2x80x9cpermanentxe2x80x9d sulfurization. As the trap""s capacity drops, the trap is xe2x80x9cfilledxe2x80x9d more quickly, and trap purge events are scheduled with ever-increasing frequency. This, in turn, increases the overall fuel penalty associated with lean engine operation, thereby further reducing the overall fuel economy benefit of enabling the operation of a xe2x80x9clean-burnxe2x80x9d feature.
In order to restore trap capacity, a trap desulfurization event is ultimately scheduled, during which additional fuel is used to heat the trap to a relatively-elevated temperature, whereupon a slightly-rich air-fuel mixture is provided for a relatively-extended period of time to release much of the stored sulfur and rejuvenate the trap. As with each purge event, each desulfurization event typically includes the further xe2x80x9cfuel penaltyxe2x80x9d associated with the initial release of oxygen previously stored in the three-way catalyst and the trap. Accordingly, the prior art teaches scheduling a desulfurization event only when the trap""s NOx-storage capacity falls below a critical level, thereby minimizing the frequency at which such further fuel economy xe2x80x9cpenaltiesxe2x80x9d are incurred.
Unfortunately, as a further impact of trap sulfurization, empirical data suggests that a trap""s instantaneous NOx-storage efficiency, i.e., its instantaneous ability to incrementally store NOx, is increasingly affected by trap sulfurization as the trap begins to fill with NOx. Specifically, while a trap""s instantaneous efficiency immediately after a trap purge event is believed to remain generally unaffected by trap sulfurization, the instantaneous efficiency begins to fall more quickly, and earlier in the fill event, with increasing trap sulfurization. Such reduced trap efficiency leads to increased instantaneous NOx emissions, even when the trap is not yet xe2x80x9cfilledxe2x80x9d with NOx.
It is an object of the invention to provide a method and apparatus for controlling a lean-burn engine of a motor vehicle which seeks to balance the respective performance impacts of increased levels of trap sulfurization and more frequent trap desulfurization in order to achieve improved engine and/or vehicle performance, such as enhanced vehicle fuel efficiency and/or reduced vehicle tailpipe emissions.
Under the invention, a method and apparatus are provided for controlling the operation of an internal combustion engine in a motor vehicle, wherein the engine generates exhaust gas including an emissions constituent, and wherein exhaust gas is directed through an emissions control device before being exhausted to the atmosphere. The method according to the invention includes determining a measure representing a performance impact of operating the engine at a first operating condition other than a near-stoichiometric operating condition, wherein the measure is based on at least one engine or vehicle operating parameter; and enabling the first operating condition based on the measure. The apparatus according to the invention includes a controller including a microprocessor arranged to determine a first measure representing a first performance impact of operating the engine at a first operating condition other than a near-stoichiometric operating condition, wherein the first measure is based on at least one engine or vehicle operating parameter; and wherein the controller is further arranged to enable the first operating condition based on the first measure.
Thus, for example, in accordance with a feature of the invention, the performance impact of continued lean-burn operation may be advantageously determined, and a lean-burn feature is advantageously enabled only when such lean-burn operation is likely to result in a positive performance impact.
In a preferred method, the performance impact is a relative efficiency or benefit calculated with reference to engine operation at the near-stoichiometric operating condition.